Medical Physics and Dosimetry

Note

all of this is kinda useless rn, scroll down for the checklist for IN-DEPTH notes!!!!!!!!!!!

The use of radioisotopes in therapeutic treatments. §

• The use of radioisotopes is common for diagnostic information about a person’s organs and treatment.
• Radiation is commonly used to weaken or destroy particular targeted cells and treat some medical conditions, especially cancer.

Single photon emission computerised tomography §

The main ways in which radioisotopes are produced for use in medicine. §

• produced locally (in the country)
• if theres too many neutrons, must beta - decay
• if theres too many protons, must beta + decay.
• australia produces it at ANSTO (australia nuclear science and technology organisation)

cyclotron §

• protons are accelerated in a spiral pathway towards the targeted material (usually something stable initially)
• protons is accepted into the nucleus, and bombards the element.

Proton therapy and Neutron therapy. §

(stolen shamelessly)

Proton therapy §

Proton therapy uses protons as the form of radiation.

• It is more precise (???) than other forms of radiotherapy.
• Protons irradiate at a lower depth, so less is scattered into healthy tissue.
• Able to adjust speed.
• Hence, we use for high required dose, but we don’t want to kill the surrounding organs!

Neutron Therapy §

• Used for specific diseases that are “radioresistant”; they are not easily removed by conventions forms of radiotherapy.
• Fast neutrons quite damaging (high quality factor), but do not damage cells equally!
  • This means for some diseases, they are more sensitive to fast neutrons than surrounding healthy tissue, meaning a lethal dose for the diseased tissues may not be lethal for the healthy tissue.
• Much more efficient, so dosage required is less (1/3 apparently) than the other forms of radiation therapy.
• Salivary gland tumours.
• Positived charge damages DNA, intending to kill target cells (usually cancers).

Radiation Dosimetry including Absorbed dose and Dose Equivalent. §

• the severity of radiation exposure depends on the amount of radiation energy that has been absorbed (E) and the mass of tissue involved (m).
• absorbed dose = energy absorbed by the tissue / mass of tissue
  • $AD=\frac{E}{m}$
  • measured in joules per kilogram ($Jk{g}^{-1}$) or grays (Gy).
• weakness of absorbed dose is that type of radiation is not accounted, which is instead accounted by dose equivalent
• dose equivalent is derived from a table of quality factors

radiation	quality factor
$\alpha$ particles	20
neutrons* (10 keV)	10
$\beta$ particles	1
$\gamma$ rays	1
X-rays	1

• the formula for dose equivalent is $DE=AD\times QF$
• variable names -> dose equivalent / absorbed dose / quality factor

$\frac{2^{x+h}-2^x}{h}$

Year 11 Physics Revision Checklist §

Science as a Human Endeavour:

• radioisotopes are used as diagnostic tools and for tumour treatment in medicine

Detecting cancer with radioactive tracers §

• Cancers formed on skin often detected by a simple external examination. HOWEVER, diagnosis of cancerous growths inside the body, a variety of radioisotopes tagged to particular drugs are used.
• The radioisotope is known as a radioactive tracer.
• These drugs, radiopharmaceuticals, can be administered by swallowing (ingestion), inhalation or injection.
• A gamma-ray camera can be used to perform a bone scan.
  • This patient has been injected with the radioisotope technetium-99m (v commonly used)
    • gamma emitter
    • half life of 6 hours
  • The camera detects the emitted gamma-rays and produces an image on a computer screen.
• The radioisotope used depends on the site of the suspected tumour.
• The body naturally distributes different elements to different organs. For example, iodine is sent to the thyroid gland by the liver. So if a radiopharmaceutical containing radioactive iodine is ingested, most of this iodine will end up in the thyroid.
• When the tracer has reached the target organ, a radiation scan is taken with a gamma-ray camera.
• An unusual pattern on the scan indicates a possible cancerous tumour. The radioisotopes used for this type of diagnosis need to be gamma-ray emitters so that the radiation has enough penetrating ability to pass out of the body to reach the detector—the gamma-ray camera.
• The isotope should have a relatively short half-life so that the patient is not subjected to any unnecessary long-term exposure to radiation.
• Radioactive tracers are also used to monitor other bodily functions. Some examples are shown in the following table.

radioactive tracer	function monitored
iodine-123	function of thyroid gland
xenon-133	function of lungs
phosphorous-32	blood flow through body
iron-59	level of iron uptake by spleen
technetium-99m	blood flow in brain, lungs and heart function of liver metabolism of bones

• nuclear power stations employ a variety of safety mechanisms to prevent nuclear accidents, including shielding, moderators, cooling

• Fuel rods: long, thin rods containing pellets of enriched uranium

• Moderator: material that slows down neutrons

  • Graphite, water, heavy water, carbon dioxide
  • Heavy water is the best but is too expensive
  • Water with dueterium, (Hydrogen-2) an isotope of hydrogen
  • Graphite and carbon dioxide contain carbon which is denser than hydrogen
  • Ensures that neutron capture occurs
  • When the neutrons collide with small nuclei, they lose most of their kinetic energy

• Control rods: material that absorbs neutrons

  • Boron steel or cadmium
  • Ensures energy release is controlled and stable, due to them controlling the amount of neutrons for fission

• Coolant: liquid to absorb heat energy that has been produced by nuclear fission, goes to cooling tower to cool down

  • e.g. water

• Radiation shield (shielding): thick concrete wall that prevents neutrons/radiation/radioactive waste escaping from the reactor

• the management of nuclear waste is based on the knowledge of the behaviour of radiation.

• A major problem facing the nuclear power industry is the disposal of the unstable radioactive waste.

• Radioactive waste products are classified as low-level, intermediate level or high-level waste.

  • Low-level waste is generated primarily from hospitals, industry and laboratories and consists mostly of tools, clothing, used wrapping material and other items that have been contaminated with radionuclides with short half-lives. Low-level waste solids are usually compacted or incinerated, then buried in shallow pits on land or at sea.
  • Intermediate-level waste typically consists of reactor components, chemical sludges, and contaminated materials from reactors that have been decommissioned. Intermediate-level wastes are solidified in bitumen or concrete, then buried or stored in deep trenches.
  • High-level waste is waste from contaminated reactor parts, as well as liquid waste from the reprocessing of spent fuel rods. This waste contains highly radioactive fission fragments and transuranic elements, and so requires special shielding during handling and transport. High-level waste remains radioactive for an exceedingly long time and needs to be stored permanently.

• By way of comparison, the activity of one tonne of uranium ore is only 8 × 10

• Uranium-235 and uranium-238 nuclei in these fuel rods have half-lives of 700000 and 4.5 billion years respectively. The numerous fission fragments have a wide range of half-lives. The fuel rods must be permanently stored in cooling ponds inside nuclear power stations in order to protect people from the radioactive emissions. In Japan, Russia and Europe, the spent fuel rods are reprocessed. The uranium is extracted and reused as nuclear fuel. This is an expensive process. Science Understanding:

• the nuclear model of the atom describes the atom as consisting of an extremely small nucleus which contains most of the atom’s mass, and is made up of positively charged protons and uncharged neutrons surrounded by negatively charged electrons

• literally the check point, no extra info abt it i dont think

• nuclear stability is the result of the strong nuclear force which operates between nucleons over a very short distance and opposes the electrostatic repulsion between protons in the nucleus

• Electrostatic forces define protons as repelling each other, but in the nucleus, an external “strong nuclear force” needs to be considered which allows protons to be very close to other protons without repelling and breaking the nucleus apart.

• The strong nuclear force is a force of attraction that holds the nucleus together and acts between every nucleon regardless of their charge. Protons are also attracted to nearby neutrons and protons.

• This force only acts over relatively short distances so for nucleons on the opposite sides of a large nucleus, this force is not significant. In a stable nucleus, there is a delicate balance between the repulsive electric force and the attractive strong nuclear force.

• some nuclides are unstable and spontaneously decay, emitting alpha, beta (+/-) and/or gamma radiation over time until they become stable nuclides

alpha decay §

no notes, assumed prior knowledge

beta decay §

• split into b+ and b-

beta - §

• electron is emitted from the nucleus of a radioactive atom.
• written as ${}_{-1}^0\beta$
• occurs if a nucleus has too many neutrons to be stable -> a neutron spontaneously changes into a proton, a beta-minus particle (${\beta }^{-}$, an electron), and an uncharged massless antimatter particle called antineutrino $\overline{v}$

beta + §

• proton spontaneously change into a neutron and emit a neutrino (v), and a positively charged beta particle.
• positively charged beta particle is called a positron, which has same properties as electron, but a positive electrical charge.

gamma decay §

• excess energy is released through a gamma ray, which is high-energy electromagnetic radiation with no mass, uncharged and travels at the speed of light.
• each species of radionuclide has a half-life which indicates the rate of decay $N=N_0{\left(\frac{1}{2}\right)}^n$, $n=\frac{t}{t_{1/2}}$ N = quantity of substance remaining $N_0$ = initial quantity of the substance t = time elapsed t1/2 = half life of the substance n = half lives elapsed
• alpha, beta and gamma radiation have different natures, properties and effects

	alpha	beta-	beta+	gamma
speed	relatively slow	up to 90% speed of light	0.9c	speed of light
charge	double pos	negative charge	positive charge	no charge
ionisation	high ionising ability	medium ionising ability	medium ionising ability	poor ionising ability
penetration	poor penetrating ability; only travels a few cm in air	1mm sheet of aluminium can stop	1mm sheet of aluminium can stop	high penetrating ability; requires a significantly thicker width of lead to stop.
mass	heavy	light	light	none

• the measurement of absorbed dose and dose equivalence enables the analysis of health and environmental risks absorbed dose = $\frac{E}{m}$, dose equivalent = absorbed dose $\times$ quality factor
• Einstein’s mass/energy relationship relates the binding energy of a nucleus to its mass defect $\Delta E=\Delta m{c}^2$
• Einstein’s mass/energy relationship also applies to all energy changes and enables the energy released in nuclear reactions to be determined from the mass change in the reaction $\Delta E=\Delta m{c}^2$
• alpha and beta decay are examples of spontaneous transmutation reactions, while artificial transmutation is a managed process that changes one nuclide into another
• neutron-induced nuclear fission is a reaction in which a heavy nuclide captures a neutron and then splits into smaller radioactive nuclides with the release of energy
• a fission chain reaction is a self-sustaining process that may be controlled to produce thermal energy, or uncontrolled to release energy explosively if its critical mass is exceeded
• nuclear fusion is a reaction in which light nuclides combine to form a heavier nuclide, with the release of energy
• more energy is released per nucleon in nuclear fusion than in nuclear fission because a greater percentage of the mass is transformed into energy